1. Field of the Invention
This invention relates to a load-sensitive level detecting device for detecting the amount of a powder, such as the toner in an electronic duplicating machine or cereal, remaining in a storage container or detecting the level of a fluid, such as the juice or coffee, in an automatic vending machine, and more particularly it relates to a load detecting device for detecting a powder or fluid by using a piezoelectric vibrator.
2. Description of the Prior Art
Devices have heretofore been known which are used to detect the level of a powder or liquid by bringing a piezoelectric vibrator in the form of a piezoelectric tuning fork or piezoelectric tuning bar into contact with the powder or liquid and thereby electrically detecting stoppage or attenuation of its vibration.
With these conventional devices, however, if used in detecting the level of a powder, particularly extrafine particles such as the toner used in duplicating machines, there is a fatal disadvantage in that because of the extrafine particles, the mass of the powder is small and the cohesion and adhesion are sufficiently high for the powder to remain stuck to the surface of the piezoelectric vibrator even if the powder level reduce below the level of installation of the piezoelectric vibrator, resulting in constraining the vibration of the piezoelectric vibrator and causing a malfunction.
To eliminate this drawback, a technique for removing the accumulated or stuck powder by causing forced vibration of the piezoelectric vibration of the piezoelectric vibrator after the powder level has been detected is disclosed in Japanese patent application No. 3378/1978 (Patent Laid-Open No. 94754/1979). However, this prior technique produces a new drawback that the arrangement of the device becomes considerably complicated.
Further, a technique for reducing accumulation of the powder by using an I-shaped tuning bar is disclosed in Japanese patent application No. 103679/1977 (Patent Laid-Open No. 37782/1979). However, this prior technique, as compared with the one using a piezoelectric tuning bar of general shape, has a drawback that the device is difficult to oscillate and inferior in its detecting ability.
Further, U.S. Pat. No. 4,314,242 dated Feb. 2, 1982 and granted to Hiroshi Kuru and Mitsuo Akiyama discloses a method of detecting the remaining amount of the toner by using a hopper whose wall surface is locally provided with a vibrator formed with electrodes on both surfaces thereof. With this prior patent, since the vibrator is formed on the wall surface of the hopper, there is no possibility of the toner sticking to the surface facing to the outside of the hopper. Therefore, the lowering of the detection accuracy due to the toner sticking to the vibrator can be reduced. However, since the inner surface of the vibrator is still in direct contact with the toner, even if the remaining amount of the toner has decreased, some toner remains stuck to the piezoelectric vibrator, thus constraining the piezoelectric vibrator. Therefore, the possibility of causing malfunction in detecting the level of the remaining amount of the toner still remains.